1. Field of the Invention
The present invention relates to an optical displacement sensor which is used for detection of a displacement of an encoder etc., and particularly, to an optical encoder.
2. Description of the Related Art
A general example of an optical encoder according to a conventional technology is an optical encoder called as a triple slit type optical encoder. This optical encoder includes a light source such as an LED (light emitting diode), a first grating which is disposed in an optical path of light from the light source, a second grating which is disposed on a scale which receives light from the first grating, a third grating on which a light which has passed through or reflected from the second grating is incident, and a photodetector which is disposed immediately after the third grating. Moreover, components excluding the scale that includes the second grating form a detecting head.
As a reflecting triple slit type optical encoder, an optical encoder shown in Utility Model Kokoku (Post Exam Application) No. Hei 6-17045 Publication is available. As shown in FIG. 10 and FIG. 11, a structure is such that a first grating 14 and a third grating 16 are formed on one transparent member, and height of the first grating 14 and the third grating 16 are aligned, and disposed in the detecting head. In this structure, setting is such that the distance from the first grating 14 up to a second grating 12, and the distance from the second grating 12 up to the third grating 16 become equal.
A principle of detection of the triple slit type optical encoder is as follows. Light emerged from the first grating 14 is diffracted at the second grating 12, then a magnified image of the second grating 12 is formed on the third grating 16, and light passed through the third grating 16 is detected at a photodetector 46.
The structure is designed such that the magnified image of the second grating 12, and the third grating 16 have the same pitch, and an amount of displacement due to a change in the intensity of light which is detected at a photodetector 46 according to a movement of a scale 40, is detected.
In the triple slit optical encoder, to align the heights of the first grating 14 and the third grating 15 according to a set value is an important point. This is because when the alignment of heights is disturbed even slightly different from the set value, a magnification of the image formed on the third grating 16 is changed and a signal amplitude of light detected at the photodetector 46 upon passing through the third grating 16 is decreased.
In the reflecting triple slit type optical encoder as shown in Utility Model Kokoku (Post Exam Application) No. Hei 6-17045 Publication, since the first grating 14 and the third grating 16 are formed on the same transparent member, it is possible to suppress a misalignment of heights by a few tens of μm at most. By this structure, the problem of deterioration of the detected signal amplitude is avoided.
On the other hand, the current trend of encoders shows significant progress in reduction of size and cost: the size of detecting heads which typically ranged between 10 to 20 mm has been reduced to merely a few mm wide. Furthermore, it might be possible that a cost of one piece of not more than 0.1 μm of resolution which is a few ten thousand yen will be reduced by one to two digits.
In such a situation, regarding packaging for facilitating the cost reduction, it has been considered that packaging will be shifted from conventional one in which a metal or ceramics is used, to one with cheaper materials such as resin molding.
In a conventional structure which is integrated for aligning the heights of the first grating and the third grating, mainly glass is used as a transparent material which forms the first grating and the third grating, and the thickness thereof is approximately 1 mm or more.
For reducing the size, when the thickness of the glass, which is a transparent member, is reduced to from one third to about one tenth and the glass is buried in a resin for molding, by a stress generated in stages of the resin molding, or by a residual stress after the manufacturing, defects such as a crack RC in a resin 60, and a breaking GW in the glass as shown in FIG. 13 may happen. Because of these defects, size-reduced encoder heads may cease to function as an encoder, and this may lead to a deterioration of reliability.
To solve this issue, a structure has been adopted in which, the member on which the first grating and the third grating are formed is divided into two members, and the two members are attached to other members the sizes being reduced to the minimum sizes required. Then it has become possible to solve the problems regarding reliability, such as a crack or breaking in the resin or glass. However, such a structure has caused too large dispersion in the signal amplitude of encoders, particularly, a decline of the signal level. Consequently, encoders suitable for mass production could not be realized.
(Experiments Related to the Tolerance of the Grating Positions)
Therefore, applicants of the this patent application carried out experiments to find out a relationship between Δzd and a signal amplitude from a photodetector, on a structure shown in FIG. 14 and FIG. 15, an optical triple-slit type encoder in which the members on which the first grating and the third grating are formed are separated. The definition of Δzd will be described later.
In FIG. 14, there is a light projecting section on a substrate 110b, and the light projecting section includes a bare-chip LED 120 as a light source, and a light transmission substrate 130 which is stacked on the bare-chip LED 120, and on which a first grating 131 is formed.
A light detecting section is provided on a substrate 110a which is separate from the substrate 110b, and the light detecting section is formed by stacking a light transmission substrate 150 on which a third grating 151 is formed on a photodetector 140. A scale 170 on which a second grating 191 is formed is disposed in parallel to a plane on which the first grating 131 and the third grating 151 are formed.
A plane parallel to the planes on which the first grating 131, the second grating 171, and the third grating 151 are formed is let to be an xy plane. An x direction is let to be a grating pitch direction of the second grating 171, a y direction is let to be a direction perpendicular to the x direction, and a z direction is let to be a direction perpendicular to the xy plane. When the scale is displaced in the x direction, since a self-image mentioned above moves on the plane on which the third grating 151 is formed, a periodic signal in a form of a quasi sine wave is obtained from the photodetector.
Here, experimental conditions are given below.
Expression 1 (will be described later): λ=650 nm, p2=20 μm, k=1
Expression 4 (will be described later): Δz0=0 mm
Effective area of first grating: 0.15 mm×0.15 mm (in FIG. 14, Wx, 1=Wy, 1=0.15 mm)
Effective area of third grating: 1.0 mm×0.5 mm (in FIG. 14, Wx, 3=1.0 mm, Wy, 3=0.5 mm)
According to the experiment of the applicants of this patent application, under these conditions, when Δzd was changed, the amplitude of the periodic signal in the form of the quasi sine wave output from the photodetector was changed as in FIG. 16. According to the experiment, when p1, p3, z1, and z2 were set such that the conditions become optimum at Δz0=0, the tolerance of Δzd for which the signal amplitude becomes ½ was ±35 μm.
On the other hand, from a manufacturing point of view, disposing by mounting a first grating 110 and a third grating 130 such that Δzd is in a range of ±35 μm, involves a high degree of difficulty in the mass production stage. Moreover, since the dimensional tolerance of structural members become strict in the direction of thickness, the z direction, it is revealed that it is very difficult to propose an optical encoder of this type at a low cost.
When the points mentioned above are put together, the conventional optical encoders in which the first grating and the third grating are formed on one transparent member have a structure which is not suitable for low-cost packaging of the resin molding etc. from a point of reliability, when the size is reduced. When the first grating and the third grating are simply formed on separate members in order to manufacture low-cost optical encoders while securing the reliability, due to the too large dispersion of mounting accuracy of the gratings in a direction of height, it is not possible to achieve encoder signals having an even amplitude, at the time of mass production.